Modern dual-circuit hydraulic braking systems for automotive applications typically include an operator-actuated brake actuation unit, such as a tandem master cylinder actuated by a booster-aided brake pedal, by which to supply a first pressurized fluid to each of a first pair of wheel brakes via a first or “primary” braking circuit, and a second pressurized fluid to each of a second pair of wheel brakes via a second or “secondary” braking circuit. The use of wholly redundant braking circuits for operating discrete pairs of wheel brakes ensures continued vehicle braking capability, notwithstanding a degradation of performance of the one of the braking circuits.
In order to achieve an “anti-lock” braking system, each braking circuit often features a normally-open electrically-operated inlet valve controlling the flow of pressurized fluid to each wheel brake, while a pressure relief line that includes a normally-closed electrically-operated outlet valve, a return pump, and a check valve controls the return of pressurized fluid from the wheel brake to the brake line upstream of the inlet valve. A “separation” or “isolation” valve, located in the brake line of each circuit upstream of the location at which the pressure relief line connects to the brake line, serves to isolate the brake line from the master cylinder during anti-lock operation.
Increasingly, such anti-lock braking systems are used in combination with wheel speed sensors in a traction control mode. The further addition of a steering angle sensor, a vehicle yaw rate sensor, and a lateral vehicle acceleration sensor enables such anti-lock braking systems to operate in an “electronic stability control” mode, wherein a braking system controller selectively energizes each circuit's electrically-operated valves when the controller identifies an opportunity to enhance vehicle stability through a selective application of the vehicle's brakes.
In order to control the fluid pressure in traction control or vehicle stability control modes, a hydraulic pump is typically placed in the pressure relief line of each circuit downstream of the outlet valve to return pressurized fluid to the circuit's brake line. The pump also serves to provide an increasing rate of fluid pressure upon the closing of the isolation valve to provide a sufficient braking system response time when operating in a traction control mode, even at a time when the brake fluid has a relatively-high viscosity due, for example, to low brake fluid temperatures.
The prior art has recognized, however, that a quicker system response is desirable when the braking system is operated in a vehicle stability control mode. By way of example, a rapid pressure build up in one or the other braking circuit is particularly desirable upon commencing vehicle stability control in order to correct oversteer or understeer conditions. Accordingly, the prior art teaches the addition of a braking circuit pre-charging function to the brake actuation unit, i.e., to the vacuum booster of the master cylinder, in order to increase system response at the time such vehicle stability control is commenced. Alternatively, an additional pre-charging pump is provided in one or both braking circuits to ensure a sufficient increasing rate of fluid pressure at the commencement of vehicle stability control enhancement.
Unfortunately, the addition of the pre-charge function to the master cylinder, or of an additional pre-charging pump to one or both braking circuits, adds significant cost, weight, and complexity to the braking system.